Field
The disclosed technology relates to a conductive copper paste for forming conducting interconnects, tracks or coating on various substrates.
Background
Conductive pastes are widely used in printed electronics; and the conductivity of the printed films highly depends on the conductive fillers used in the paste. Copper, with its high conductivity, is commonly used as filler, but it is easily oxidized to form copper oxide, which has a conductivity one millionth to that of copper. To prevent copper from oxidizing, copper particles are passivated by a coating, which is removed during sintering.
The filler's size and morphology also influences the product quality and processing conditions. For a paste containing micron-sized or finer copper flakes as fillers, sintering has to be conducted at a high temperature under an inert environment. This makes thermally-sensitive material not suitable to be used as substrate. Copper nanoparticles exhibiting melting point depression allows the paste to be sintered at a lower temperature. The melting point depression allows a thermally-sensitive substrate to be used; however, the printed film has a high tendency to crack. Copper flakes are also preferred over spherical particles as flakes provide more contacting points among fillers for conduction.
For synthesizing copper nanoparticles for paste production, chemical reduction can be applied to produce monodispersed nanoparticles. Different copper sources and reagents are used to produce nanoparticles in different sizes. To prevent metal nanoparticles from aggregating and oxidizing, the nanoparticles can be coated with a hydrophobic (US 2008/0138643) or hydrophilic (US 2007/0180954) capping agent. The capping process is conventionally obtained by pre-mixing excess capping agent with a metal precursor to form a homogeneous solution, followed by reduction. The existing process is characterized by a low product concentration (100 mM) and low recovery. It is often not feasible to change the capping agent.